1. Field of the Invention
The present invention relates to an image forming apparatus provided in a facsimile machine, a copy machine, or a printer machine (or a Multi Function Peripheral (MFP) of these machines). In particular, the present invention relates to an image forming apparatus using a contact and transfer method and including an electrophotographic printer unit having a photoconductive drum and a transfer roller, which rotate while making contact with one another, wherein paper is introduced into a contact portion between the photoconductive drum and the transfer roller and a toner image is transferred onto the paper.
2. Description of the Related Art
In an electrophotographic printer unit, an electrostatic latent image is formed on a surface of a photoconductive drum. A developing device develops the electrostatic latent image to form a toner image. Then, paper, which has been introduced in synchronism with forming the toner image, is nipped in a nip portion between the photoconductive drum and a transfer roller, and the toner image is transferred onto the paper. A paper transportation guide is arranged at a position located upstream of the nip portion in a paper transportation direction and in proximity of the nip portion. Thus, a leading edge of the paper is introduced accurately into the nip portion. In this case, when the leading edge of the paper, which is guided by the paper transportation guide, hits the transfer roller first, toner on the surface of the photoconductive drum may scatter due to an electric field between the photoconductive drum and the transfer roller. Therefore, the paper transportation guide is positioned appropriately so that the leading edge of the paper hits the surface of the photoconductive drum first and then the paper is introduced into and nipped by the nip portion accompanying the rotation of the photoconductive drum.
In a first conventional apparatus, a paper transportation guide is fixed on an apparatus main body or mounted integrally on a cover, which is opened and closed for removing jammed paper. In a second conventional apparatus, to prevent toner from scattering, a paper transportation guide guides paper to a position located in proximity of a nip portion. In addition, to maintain ease of maintenance work performed when removing and inserting a transfer roller to replace the transfer roller, the paper transportation guide is held removably with respect to a positioning member of the photoconductive drum.
However, as described above, when the paper transportation guide is fixed on the apparatus main body, or when the paper transportation guide is mounted integrally on the cover, the removable photoconductive drum or a drum unit (process unit) and the transfer roller are not precisely positioned with respect to the paper transportation guide. Therefore, the scattering of the toner resulting from a position where the paper is introduced as described above is not sufficiently prevented. In the second conventional apparatus, although the paper transportation guide is held removably with respect to the positioning member of the photoconductive drum, the paper transportation guide is not positioned directly with respect to the photoconductive drum. In the same manner, the paper transportation guide is not positioned directly with respect to the transfer roller. Therefore, a relative positional relationship of the photoconductive drum, the transfer roller, and the paper transportation guide is difficult to be appropriately set. Thus, to maintain a high printing (recording) precision, an improvement is desired.